Some of the best parts of houses are details that you wouldn't expect. Rounded walls and nooks and crannies are all details that add visual interest but are tricky to figure out how to decorate around. These could have been decisions made by architects or builders, or attributes added by previous owners. Whatever the situation, a room oddity can be a difficult feature to figure out how to fit into your décor. Your first decision is whether you want to work with it or cover it up.
If your desire is to go with what you've got, go ahead and make a bold statement - go big with color. Say you have a weird window configuration, or some interesting architectural details on your ceiling or walls. Choose a bold paint scheme to draw the eye to it, making it the focal point. Contrasting colors are a great way to do this. But maybe it's a feature that just doesn't fit with what you had in mind. If so, you should stick to a neutral palette, which will help these features blend into the background. Then, create your focal point of the room in a different area, assuring that your eye won't be drawn to the oddities.
Sometimes your oddity is an eyesore that you just want to make go away. Curtains are a great way to cover up something that can't be removed. Or, find a large piece of art that blocks the view. Multi-panel screens are popular décor pieces that can be easily moved if the area needs to be accessed. Then use the space behind it as extra storage. Freestanding bookcases are also good blockers that can be used as storage or to display accessories or collections.